Method of simultanously reducing CO2 and SO2 emissions in a combustion installation

ABSTRACT

The method of simultaneously reducing carbon dioxide (CO 2 ) emissions and sulfur dioxide (SO 2 ) emissions produced by the combustion of carbon-containing matter in a hearth consists in injecting into the hearth a calcium-based agent, a fraction of which absorbs SO 2  after decarbonization, and then, after the flue gases have been subjected to intermediate cooling, in causing them to transit via a first reactor and in putting them in contact therein with the other fraction of the absorbant that has not reacted with SO 2  so as to capture CO 2  from the flue gases by carbonization, then, in a separator, in extracting the solids contained in the flue gases output from the first reactor so as to subject them to heat treatment in a second reactor in order to extract CO 2  therefrom by decarbonization and in order to recycle the resulting regenerated CO 2  absorbant to the first reactor.

[0001] The invention relates to installations for burningcarbon-containing matter, e.g. fossil fuels or waste, in a hearth, inparticular a hearth operating as a fluidized bed. More particularly, itrelates to a method of reducing both carbon dioxide (CO₂) emissions andalso sulfur dioxide (SO₂) emissions in the flue gases produced by thistype of installation.

BACKGROUND OF THE INVENTION

[0002] It is known that the combustion of fossil fuels such as coal orwaste produces gaseous emissions of CO₂ and of SO₂, and that injectingcalcium carbonate (CaCO₃) into the hearth makes it possible to reduceSO₂ emissions in situ. Unfortunately, such reduction in SO₂ isaccompanied by production of CO₂ in addition to the CO₂ coming from thecombustion of the carbon-containing matter. CO₂ is a “greenhouse” gaswhich would appear to contribute to global warming.

[0003] In addition, injecting calcium carbonate for desulfurizing theflue gases suffers from the drawback of producing, in the hearth, alarge quantity of ash that is too rich in calcium sulfate (CaSO₄) and inlime (CaO) to be easy to recycle.

OBJECTS AND SUMMARY OF THE INVENTION

[0004] An object of the invention is to remedy those drawbacks.

[0005] To this end, the invention provides a method of simultaneouslyreducing carbon dioxide emissions and sulfur dioxide emissions producedby the combustion of carbon-containing matter in a hearth, said methodconsisting in injecting into the hearth a calcium-based agent, afraction of which absorbs SO₂ after decarbonization, and then, after theflue gases have been subjected to intermediate cooling, in causing themto transit via a first reactor and in putting them in contact thereinwith the other fraction of the absorbant that has not reacted with SO₂so as to capture CO₂ from the flue gases by carbonization, then, in aseparator, in extracting the solids contained in the flue gases outputfrom the first reactor so as to subject them to heat treatment in asecond reactor in order to extract CO₂ therefrom by decarbonization andin order to recycle the resulting regenerated CO₂ absorbent to the firstreactor.

BRIEF DESCRIPTION OF THE DRAWING

[0006] An implementation of the method of the invention is described inmore detail below with reference to the sole figure.

MORE DETAILED DESCRIPTION

[0007] Combustion of carbon-containing matter such as fossil fuels orwaste produces gaseous emissions of CO₂ and of SO₂. In known manner,combustion in a fluidized bed, for example, makes it possible to obtaineffective desulfurization of the flue gases when an absorbant based oncalcium such as calcium carbonate is injected into the hearth, suchdesulfurization taking place by means of the following reactions:

[0008] CaCO₃ ->CaO+CO₂ (decarbonization)

[0009] CaO+SO₂+1/20 ₂ ->CaSO₄

[0010] Such fluidized bed combustion thus suffers from the drawback thatit generates CO₂ in addition to the CO₂ that results from burning theorganic carbon of the fuel.

[0011] It is also known that CO₂ is a “greenhouse” gas whoseconcentration in the atmosphere is increasing, which could contribute toglobal warming. A second drawback of such fluidized bed combustion liesin the quantity of ash produced that is rich in CaSO₄ and in CaO, whichcould limit the use of such ash.

[0012] The invention makes it possible to minimize those drawbacksfirstly by significantly reducing CO₂ emissions coming from thedecarbonization of the calcium carbonate, and from the oxidation of theorganic carbon, and secondly by making it possible for the absorbant tobe regenerated. The basic idea of the invention is to use the surplusCaO in the ash to capture CO₂ from the flue gases.

[0013] As shown in the figure, calcium carbonate 2 is injected as anabsorbing agent into the hearth 1, which, in this example is acirculating fluidized bed hearth. This hearth, which operates at atemperature of about 850° C., is provided with a particle separator ofthe cyclone type which returns the particles to the bottom of thehearth. The flue gases produced at the output of the hearth are chargedwith CaO and they pass through a recuperator boiler 3 so as to lowertheir temperature to within the range 800° C. to 400° C., and typicallyto 650° C. The resulting cooled flue gases charged with CaO penetrateinto the reactor 4 into which the regenerated absorbing agent 5 isre-injected as indicated below. The regenerated absorbing agent 5 ismade up in part by the fraction of the absorbing agent 2 that has notreacted with SO₂ in the hearth 1. While in the reactor 4, the flue gasesare put in contact with the regenerated absorbing agent and with the CaOcontained in the incoming flue gases so as to capture CO₂ contained inthe flue gases, by means of the following reaction:

[0014] CaO+CO₂ ->CaCO₃ (carbonization)

[0015] The solids present in the outgoing flue gases that leave thereactor 4 are extracted in part in a gas/solid separator 6, e.g. acyclone.

[0016] For example, the reactor 4 and the associated separator 6 may bea re-circulating fast fluidized bed reactor characterized by a gastransit time dependent on the desired CO₂ capture efficiency. Typicallythe CO₂ capture efficiency lies in the range 20% to 80%, with gastransit times shorter than 10 seconds, and with solid transit times ofseveral minutes.

[0017] Under the separator 6, the reactor 4 may include a particle heatexchanger 18 and an outlet valve 19 having an adjustable flow rate,making it possible to adjust the temperature in the reactor 4 so as tooptimize the exothermic CO₂ absorption reaction and the temperature inthe reactor 11, the re-circulation to the reactor 4 serving to increasethe time during which the CaO transits through the reactor 4. Moreparticularly, the valve 19 is disposed between the separator 6 and theparticle heat exchanger 18 in which a fraction of the solids output fromthe separator 6 is cooled before being re-injected into the reactor 4.

[0018] On leaving the separator 6, the flue gases with part of the CO₂content removed go through a second recuperator boiler 8 in which theyare cooled conventionally to about 120° C., and they then go through afinal dust filter 9 before they are released into the atmosphere via achimney at C. The solids 10 collected in the dust filter 9 are in partrecycled and mixed with the solids 7 output by the separator 6, in areactor 11 heated to a temperature higher than the temperature of thereactor 4, and typically higher than 800° C., so as to enable CO₂ to bereleased by means of the following reactions:

[0019] CaCO₃ ->CaO+CO₂ (decarbonization)

[0020] By further increasing the temperature in the reactor 11, it ispossible to cause SO₂ to be released as well.

[0021] In a variant, by pre-treating the solids 7 and 10, e.g. withwater in an enclosure 17, before they are sent to the reactor 11, it ispossible to reduce the operating temperature of the reactor 11.

[0022] The gas 12 produced by the reactor 11 is essentially a mixture ofCO₂ and of SO₂. These two components can be separated from each other at12 for subsequent use or for underground storage.

[0023] The solids 13 extracted from the reactor 11 and containing a highproportion of CaO are optionally treated in an enclosure for the purposeof improving their reactivity for CO₂ and SO₂ capture, e.g. by addingwater or water vapor or by adding a reaction promoter such as sodiumsalts, before they are returned to the reactor 4 or to the hearth 1. Inaddition, in the enclosure 14, the solids may be cooled so as tofacilitate transporting them. It is therefore in the reactor 11 that theCO₂ absorbant is regenerated so as to be recycled to the reactor 4.

[0024] In order to minimize the consumption of calcium carbonate 2, asmall fraction of the solids 13 is recycled to the hearth 1. A fraction16 of the ash usually extracted from the fluidized bed at the bottom ofthe hearth 1 may be re-injected into the reactor 11.

[0025] The reactor 11 is preferably equipped with a bleed outlet 20making it possible to extract the surplus solids if necessary in orderto regulate the solids content of the reactor 4. The grain-size of theabsorbing agent 2 is adjusted by grinding so as to optimize the quantityof absorbing agent contained in the flue gases that are released fromthe hearth 1.

[0026] The absorbing agent 2 injected into the hearth 1 mayadvantageously be dolomite containing magnesium carbonate, which makesit possible to reduce the operating temperature of the reactor 11, andto reduce the decarbonization energy required. More generally, the agent2 may be an alkaline earth which behaves in the same way as calciumcarbonate but at different temperatures.

[0027] The invention is not limited to fluidized bed combustion. It isapplicable to various combustion modes, in particular using powderedcoal, and more generally to treating flue gases containing CO₂ producedby a combustion hearth into which an absorbant is injected.

[0028] The method of the invention is applicable to an existinginstallation having a recuperator boiler 3 by making a relatively simpleadaptation to the boiler so as to couple it to the reactor 4.

1/ A method of simultaneously reducing carbon dioxide (CO₂) emissionsand sulfur dioxide (SO₂) emissions produced by the combustion ofcarbon-containing matter in a hearth, said method consisting ininjecting into the hearth a calcium-based agent, a fraction of whichabsorbs SO₂ after decarbonization, and then, after the flue gases havebeen subjected to intermediate cooling, in causing them to transit via afirst reactor and in putting them in contact therein with the otherfraction of the absorbant that has not reacted with SO₂ so as to captureCO₂ from the flue gases by carbonization, then, in a separator, inextracting the solids contained in the flue gases output from the firstreactor so as to subject them to heat treatment in a second reactor inorder to extract CO₂ therefrom by decarbonization and in order torecycle the resulting regenerated CO₂ absorbant to the first reactor. 2/The method of claim 1, in which the flue gases output from the separatorare sent to a dust filter, and part of the solids extracted from thedust filter are mixed with the solids extracted from the separatorbefore being subjected to heat treatment in the second reactor. 3/ Themethod of claim 1, in which the flue gases output from the separator arecooled in a recuperator boiler before being sent into the dust filter.4/ The method of claim 1, in which, before the solids are sent to thesecond reactor, their temperature is reduced in a first enclosure bytreatment with water. 5/ The method of claim 1, in which a fraction ofthe solids output from the second reactor is re-injected into thehearth. 6/ The method of claim 1, in which a fragment of the ash takenfrom the hearth is sent to the second reactor. 7/ The method of claim 1,in which the solids output from the second reactor are treated in asecond enclosure so as to improve their reactivity before beingre-injected into the hearth or into the first reactor. 8/ The method ofclaim 7, in which the solids are reactivated in the second enclosure byadding alkaline salts. 9/ The method of claim 1, in which a fraction ofthe solids output from the separator is re-injected into the firstreactor via a valve having an adjustable flow rate. 10/ The method ofclaim 1, in which a fraction of the solids output from the separator iscooled in a solid particles heat exchanger and is then re-injected intothe first reactor. 11/ The method of claim 1, in which the first reactoris a fluidized bed reactor. 12/ The method of claim 1, in which thehearth is a circulating fluidized bed hearth. 13/ The method of claim 1,in which the absorbant is calcium carbonate. 14/ The method of claim 1,in which the absorbant is an alkaline-earth, and in particular dolomite.